This invention relates to an ultrasonic machining method and apparatus, and particularly to an ultrasonic abrasive grain machining method and apparatus of machining fine ceramics or the like, or to an ultrasonic discharge machining method and apparatus of machining hard materials.
In a conventional abrasive grain machining method or electric discharge machining method utilizing ultrasonic vibration, a machining solution is used at room temperature under atmospheric pressure. Therefore, when the amplitude of ultrasonic vibration is increased to improve the machining efficiency, cavitation (formation of substantially vacuum bubbles) is liable to occur, thus engraving the machined surface in such a manner that small pits are linked therein so that the working accuracy or smoothness of the finished surface is reduced.
The reason for this is as follows: When the vibrating tool is quickly moved up and down while ultrasonic vibration is given to the solution, the solution tends to flow following the movement of the tool. If it flows at high speed, then pressure difference occurs in the solution; i.e., its part of higher flow speed is lower in pressure (Bernoulli's law).
The ultrasonic vibration is, in general, of the order of 20 KHz. If, in this case, the amplitude is 30 .mu.m (full amplitude), then a considerably large acceleration of the order of 450.times.10.sup.3 m/s.sup.2 occurs at the maximum amplitude point, resulting in the occurrence of cavitation which is great beyond comparison with that which occurs with a pump or cascade of blades.
Vacuum cavities formed by cavitation produce a great force of suction when collapsed. The force of suction thus produced may provide an ultrasonic cleaning effect; however, in a machining operation, it will suck weak brittle crystal particles out of the grain boundary, as a result of which the machined surface is made uneven, or pitted.
Conventionally, in order to overcome the above-described difficulty, the amplitude is decreased thereby to decrease the machining speed, or nothing has been done; that is, the drawback is accepted as unavoidable.
When cavitation occurs as described above, in an ultrasonic machining operation, a brittle material such as semi-sintered material will be greatly engraved.
On the other hand, in an ultrasonic discharge machining operation, a finely machined surface can be obtained with high efficiency; however, arc marks are liable to be formed at the point where cavitation occurs, which obstructs practical application of the operation.